The invention relates to cardiology, and more specifically relates to myocardial perfusion studies carried out using magnetic resonance (MR) imaging.
Cardiologists use myocardial perfusion studies for the noninvasive diagnosis of coronary artery disease ("CAD"). In a conventional myocardial perfusion study, the patient is injected with a radioisotope tracer (e.g. .sup.201 T1 or .sup.99m Tc) and a scintillation camera is used to form a three-dimensional image of the tracer distribution within the patient's heart. (This imaging technology is known as "nuclear medicine".) This image permits a diagnostician to differentiate regions of the myocardium that receive normal blood flow (regions that are normally "perfused"), from those with abnormally reduced blood flow (caused by CAD). Monitoring myocardial perfusion under predetermined conditions (e.g. with the patient at rest and/or under exercise- or pharmacologically-induced stress) allows cardiologists to diagnose the existence and severity of CAD.
MR imaging is capable of spatial resolution that is an order of magnitude greater than nuclear medicine imaging, offering the possibility of greater sensitivity and specificity for the detection and delineation of regional myocardial perfusion defects. For this reason, doctors have long sought to carry out myocardial perfusion studies using the MR modality.
In such attempts, the patient's blood has been labeled using a T1-lowering contrast agent (such as Gd-DTPA), which makes the blood appear bright in a T1-weighted MR image. In this way, differences in regional myocardial perfusion can be identified by differences of signal intensity within the imaged myocardium. Normally perfused tissue will show signal enhancement with the arrival of T1-shortening contrast agent; poorly perfused tissue will show no enhancement or enhancement that is diminished and delayed.
Attempts to image myocardial perfusion using the MR modality have had limited success and have not yet been put into clinical practice. One reason for this has been that researchers have focussed upon quantification of myocardial bloodflow. To quantify myocardial bloodflow using first-pass enhancement techniques, the MR signal must be sampled rapidly enough to produce about one image per second. (Such a high temporal resolution is required to characterize the enhancement curve with accuracy sufficient to estimate myocardial bloodflow.)
Even with modern high-performance MR imagers, the price of achieving such high temporal resolution is a reduction in spatial resolution and signal-to-noise ratio, and a reduction in the volume that is imaged (such a volume reduction is known as a reduction in "coverage"). As a result, the quality of MR myocardial perfusion studies has been unacceptable for diagnostic purposes. So, too, it has been impossible to cover the entire heart in a single study, severely limiting the value of the technique as a means of noninvasively detecting CAD.
It would be advantageous to provide a method for carrying out a myocardial perfusion study using MR imaging, in which the reconstructed MR images had sufficiently good spatial resolution, sufficiently good signal-to-noise ratio, and sufficiently good contrast between normal and abnormal tissue as to be acceptable for diagnostic purposes, and in which the entire heart could be imaged in a single study.
One object of the invention is to provide a method for carrying out a myocardial perfusion study using MR imaging, in which the reconstructed MR images can have a high spatial resolution and a high signal-to-noise ratio.
Another object is to provide a method for carrying out a myocardial perfusion study using MR imaging, in which reconstructed MR images of the entire heart can be produced on the basis of a single study.
Another object is, in general, to improve on known methods of this general type.
The invention proceeds from the realization that an MR myocardial perfusion study need not have high temporal resolution in order to be diagnostically valuable. Rather, if acquisition of MR image data is accurately timed to take place throughout the several seconds it takes for a bolus of MR contrast agent to make its first pass through the heart, it is possible to produce MR myocardial perfusion images of high diagnostic value.
In accordance with the invention, a bolus of MR contrast agent is administered to the patient, and MR data acquisition occurs when that bolus makes its first pass through the heart. (Advantageously, the acquisition begins when or shortly before the bolus reaches the heart, and ends when or shortly after the bolus leaves the heart.) The acquisition is gated to the patient's cardiac cycle. In the preferred embodiment, lines of MR data are acquired only during the diastolic phase in successive cardiac cycles, and advantageously while the patient holds his or her breath. These measures minimize cardiac wall motion during MR data acquisition and thereby maximize the spatial resolution of the reconstructed MR images of the heart.
Advantageously, and in accordance with the preferred embodiment, the T1-weighting of the MR signal is emphasized. This can be accomplished by using magnetization preparation of the inversion-recovery or saturation-recovery types. This results in an especially advantageous reconstructed MR image, because the contrast agent is more prominently displayed in the MR image, making it easier to distinguish between perfused and unperfused myocardial regions.
In the preferred embodiment, MR data acquisition is carried out using an MR pulse sequence of the T1-weighted, three-dimensional, gradient-echo type. Three-dimensional pulse sequences make it possible to image the heart with contiguous thin sections. Gradient-echo pulse sequences make it possible to collect many lines of MR data in a short time span (because such sequences can have a very short repetition time TR).
As stated above, MR data are acquired while the bolus of the MR contrast agent is making its first pass through the heart, and it is therefore necessary to make sure that data acquisition occurs at the correct time. At this writing, the inventor believes there are two ways in which this can be efficiently accomplished. The first is to experimentally establish the time delay between the injection of the MR contrast agent and the arrival of the MR contrast agent at the left ventricle of the patient's heart. This delay can then be used to start three-dimensional data acquisition after injection of the MR contrast agent. In practice, this time delay can be measured by injecting the patient with a small dose of MR contrast agent and using a two-dimensional acquisition having a high temporal resolution to determine when the MR contrast agent has reached the left ventricle. The second is to acquire, reconstruct and display two-dimensional images in real time after the contrast agent has been injected, and to switch to a three-dimensional acquisition when the contrast agent reaches the left ventricle. As of this writing, the first method has been used; work on the second method is ongoing.